Anxiety, depression and alcohol and drug dependency cases might be more than twice as high as researchers have come to believe, a study published Thursday in the journal Psychological Science finds, with 41 percent of young adults experiencing major depression, half suffering an anxiety disorder and nearly one in three exhibiting alcohol dependence by the age of 32.

But coming at a time when psychiatrists are updating the manual they use to diagnose mental illness, the possible doubling of the potential patient population is expected to spark lively debate.

Mental health professionals, insurance companies and drug manufacturers have a profound interest in knowing how many of us can be said to suffer mental illnesses such as depression, anxiety disorders or addiction in our lifetime. These conditions are considered common. But it would be nice to know just how common they really are.

Researchers have long sought to gauge the prevalence of such illnesses by asking middle-aged adults to recall how much or often they drank alcohol in college, whether they were ever extremely sad for more than three weeks straight as an adolescent, or whether bouts of worry ever significantly hampered their job performance or daily activities when they were launching their careers.

It seems a lot of bad patches get forgotten in the process.

Such "retrospective" studies as the U.S. National Comorbidity Survey have found that between 18 and 32 year old, 18 percent of Americans suffer at least one episode of clinical depression, and between 6 percent and 17 percent of that age group could be diagnosed with alcohol dependence. It also found that between one-quarter and one-third of Americans suffered from some anxiety disorder in that span of life.

Some think a better way to assess how commonly these illnesses occur would be to recruit a group of infants and ask them at regular intervals into adulthood whether they've had specific symptoms that define depression, anxiety disorder and substance abuse. When a group of U.S., British and New Zealander researchers did that with 1,000 New Zealanders, and kept up with them until age 32, they believe they got a more accurate accounting of the incidence of common mental illnesses. And they were a lot more common than had been suspected.

That surprising finding is likely to prompt some experts in psychiatry to ask whether the profession has set the thresholds for defining those illnesses too low or drafted them too loosely. If the definitions of these conditions are too broad, the result could be over-diagnosis, along with over-medication, over-treatment and pathologizing of emotions that are normal and manageable. Drug companies could enjoy a boom of new customers, and could exploit the opportunity to fudge diagnostic borders to include an ever-widening slice of the population in their potential markets.

If the new survey is an accurate portrayal of how often children and younger adults suffer mental illness, however, it is clear that depression, anxiety and alcohol dependence are woefully under-diagnosed and under-treated.